112 



ANCIENT PLANTS 



the Cordaitece, and comes very close to an intermediate 

 group of fossils to be considered in the next chapter. 



To enter into the detailed anatomy of the seeds 

 would lead us too far into the realms of the specialist, 

 but we must notice one or two points about them. 

 Firstly, their very large size, for ripe seeds of Cycas are 

 as large as peaches (and peaches, it is to be noted, are 

 fruits, not seeds), and particularly the large size they 



attain before they are fer- 

 tilized and have aft embryo. 

 Among the higher plants 

 the young seeds remain very 

 minute until an embryo is 

 secured by the act of fer- 

 tilization, but in the Cycads 

 the seeds enlarge and lay 

 in a big store of starch in 

 the endosperm before the 

 embryo appears, so that in 

 the cases in which fertiliza- 

 tion is prevented large, 

 sterile "seeds" are never- 

 theless produced. This 

 must be looked on as a 

 want of precision in the 

 mechanism, and as a waste- 

 ful arrangement which is undeniably primitive. An even 

 more wasteful arrangement appears to have been com- 

 mon to the "seeds" of the Palaeozoic period, for, though 

 many fossil "seeds" are known in detail from the old 

 rocks, not one is known to have any trace of an embryo. 

 A general plan of the Cycas seed is shown in fig. 76, 

 which should be compared with that of Ginkgo (fig. 68). 

 The large size of the endosperm and the thick andi 

 complex seed -coats are characteristic features of both 

 these structures. Another point that makes the Cycad 

 seeds of special interest is the fact that the male cells 

 (as in Ginkgo] are developed as active, free-swimming 



SC: 

 Fig. 76. Seed of Cycas cut open 



n, The nucellus, fused at the level / to 

 the coat c\ sc, stony layer of coat; p.c, 

 pollen chamber in apex of the nucellus; 

 S, "spore", filled with endosperm, in 

 which lies the embryo e. 



